MVDC distribution

In recent times, the most advanced navies in the world are adopting the AES concept for their new vessels through the installation of Medium Voltage Alternate Current (MVAC) IPSs. To successfully design such ships navy designers have drawn largely from the knowledge gained in the merchant field. Due to that, in such ships the design effort has been put mainly on achieving high levels of reliability and to improve mission capabilities, starting from a well-known design base. Examples of the most recent naval vessels built using AES concept are UK Navy Type 45 and the abovementioned aircraft carrier HMS Queen Elizabeth, and the French/Italian FREMM frigates. Moreover, the use of hybrid-propelled ships (which have installed onboard both mechanical and electrical propulsion systems) is foreseen for all the new ships planned for acquisition by the IT Navy, exploiting the interest of Navies in AES concept. However, the nowadays adoption of MVAC systems is only a starting point for navies, because the struggle in achieving ever higher performance is pushing the research (on ship’s power systems) towards new concepts, such as the Medium Voltage Direct Current (MVDC) distribution system. [44] [52] [53] [54]. In recent years, the research has been focused onto this topic mainly because of the financing of US Navy, whose interest in such a technology is major. Such a high interest is due to the advantages that can be given by DC distribution to naval applications. Still, some relevant issues are present, whose solving require both academic and industrial research effort.

A review of the pros of DC power distribution over AC one is given in the following:

a. Simplifying connection and disconnection of different types and sizes of power generation and storage devices;

b. Reducing the size and ratings of switchgear;

c. Eliminating large low-frequency (50 Hz or 60 Hz) transformers;

d. Limiting and managing fault currents and enabling fast system reconfigurations;

e. Eliminating reactive voltage drop;

f. Reducing power system weight by using high speed generators;

g. Enabling higher power ratings for a given cable size;

h. Enabling active power flow management, especially during transients and in emergency conditions;

i. Reducing fuel consumption by allowing variable speed prime mover operation;

j. Improving efficiency when energy storage is used;

k. Rationalizing power conversion stages;

l. Eliminating the need for phase angle synchronization of multiple sources and loads.

Most of these advantages are related to the high amount of electronic power conversion systems present in an MVDC system. In fact, conversion systems are needed in DC power system to allow their proper operation (as an example, in DC no simple static machines to change the voltage level are available). However, such a pervasive electronic power conversion presence leads to the main technical issue of MVDC power systems: the Constant Power Loads voltage instability issue. Such an issue has been already discussed in case of AC systems, in chapter 2.3.4 (page 45 ff.). In DC systems such issues is also present and depends on similar causes. Several research activities are aimed at solving such issues, applying different approaches. A good reference to such an issue, and methods to solve it, can be found in [55], together with relevant bibliography. In addition to that, MVDC systems present other relevant issues, which need to be solved prior their common adoption as onboard systems:

a. Difficulty in extinguishing DC arcs in the absence of a voltage or current zero crossing (issues in building appropriate breakers).

b. Definition of an effective grounding strategy to provide crew electric safety.

c. Lack of an established industrial base, being MVDC systems an insignificant commercial market nowadays.

In particular, the last point is one of the most significant obstacles to the adoption of MVDC power systems. In fact, the absence of industrial partners able to supply the components needed to install an MVDC system leads designer to generally ignore such a solution for onboard distribution, which in turn discourages suppliers’ investments in the MVDC sector.

Luckily, in order to exit from this impasse situation some major power components suppliers are starting investing in industrial research to put on the market products for the MVDC systems, because they see in such distribution systems a business opportunity.

To allow comprehending what might be an MVDC power system, a possible functional block diagram is depicted in Figure 14 [44]. The functional blocks can be defined as follows:

• Shore power interface: a power source that adapts electric energy from the utility system on shore to MVDC power system (e.g. transformer + AC/DC interface converter).

• Power generation: a power source that converts prime energy from fuel into electric energy, hereinafter adapted to MVDC (e.g. prime mover + generator + AC/DC interface converter). It may also be a fuel cell system.

• Energy storage: a system capable to store energy, taken from the system, in order to supply it back when needed (e.g. super-capacitor, battery, flywheel), used to face transient power unbalances and as an active filtering unit to improve Power Quality.

• Pulsed load: a load center that draws intermittent pulses of power from the power system, (e.g. electromagnetic aircraft launch system, rail gun, and free electron laser), generally a load specific to military area.

• Propulsion: a load center constitute by electric motors, supplied from the DC distribution bus through variable speed drive inverters, used to achieve the ship movement and maneuverability.

• Ship service: a load center that primarily draws power from the system to ship services (e.g. hotel load).

• Dedicated High Power Load: a load center that draws high amount of power from the power system (1 MW or more of power in steady-state operation) (e.g. military radar, large thruster, compressor).

• Ship-wide power and energy management control: PMS conceived to maximize the continuity-of-service of vital loads during reconfiguration operations, optimizing the power flows throughout the ship.

• System Protection: DC system protection is achieved through a combination of converter control and other DC circuit breaking devices (e.g. solid-state DC breakers).

• MVDC bus: the ensemble of busbars and breakers of the MVDC system, allowing its division in sub-sections.

As aforementioned, the MVDC power system foresees the extensive use of power converters [56]. Indeed, each electrical power source and each load must be interfaced to the MVDC bus via converters, as clearly shown in the hypothetical notional MVDC power system with radial architecture shown in Figure 15. This enables innovative functionalities to be integrated in the

converters, such as short circuit protection integrated directly into the converter, or fast reconfiguration, now never used in industrial applications.

Figure 14 - Functional block diagram of MVDC power system [44].

Figure 15 - MVDC radial distribution [44].

Such an innovative power system requires new tools to be designed due to the absence of prior knowledge. Moreover, components never used before are foreseen to be installed in MVDC systems. This leads to the need of a design process that is able to infer the impact of all these new components on the overall system and which can help designers in comprehending how such systems are supposed to behave. In this regard, the innovative design process proposed in this thesis work can help designers in building a ship endowed with an MVDC power system.